<template>
  <transition :name="transitionName">
    <div
      v-show="visible"
      :style="customStyle"
      class="back-to-ceiling"
      @click="backToTop"
    >
      <svg
        width="16"
        height="16"
        viewBox="0 0 17 17"
        xmlns="http://www.w3.org/2000/svg"
        class="Icon Icon--backToTopArrow"
        aria-hidden="true"
        style="height: 16px; width: 16px"
      >
        <path
          d="M12.036 15.59a1 1 0 0 1-.997.995H5.032a.996.996 0 0 1-.997-.996V8.584H1.03c-1.1 0-1.36-.633-.578-1.416L7.33.29a1.003 1.003 0 0 1 1.412 0l6.878 6.88c.782.78.523 1.415-.58 1.415h-3.004v7.004z"
        />
      </svg>
    </div>
  </transition>
</template>
<script>
import {
  defineComponent,
  onBeforeUnmount,
  onMounted,
  reactive,
  ref,
  toRefs,
} from "vue";
// demo
//  :custom-style="{
//   right: "50px",
//         bottom: "50px",
//         width: "40px",
//         height: "40px",
//         "border-radius": "4px",
//         "line-height": "45px",
//         background: "#e7eaf1",
//        }"
//         :visibility-height="300"
//         :back-position="50"
//         transition-name="fade"
export default defineComponent({
  props: {
    visibilityHeight: {
      type: Number,
      default: 400,
    },
    backPosition: {
      type: Number,
      default: 0,
    },
    customStyle: {
      type: Object,
      default: function () {
        return {
          right: "50px",
          bottom: "50px",
          width: "40px",
          height: "40px",
          "border-radius": "4px",
          "line-height": "45px",
          background: "#e7eaf1",
        };
      },
    },
    transitionName: {
      type: String,
      default: "fade",
    },
  },
  setup(props) {
    const visible = ref(false);
    const isMoving = ref(false);

    const state = reactive({
      interval: null,
    });

    onMounted(() => {
      window.addEventListener("scroll", handleScroll);
    });
    onBeforeUnmount(() => {
      window.removeEventListener("scroll", handleScroll);
      if (state.interval) {
        clearInterval(state.interval);
      }
    });
    const handleScroll = () => {
      visible.value = window.pageYOffset > props.visibilityHeight;
    };
    const backToTop = () => {
      if (isMoving.value) return;
      const start = window.pageYOffset;
      let i = 0;
      isMoving.value = true;
      state.interval = setInterval(() => {
        const next = Math.floor(easeInOutQuad(10 * i, start, -start, 500));
        if (next <= props.backPosition) {
          window.scrollTo(0, props.backPosition);
          clearInterval(state.interval);
          isMoving.value = false;
        } else {
          window.scrollTo(0, next);
        }
        i++;
      }, 16.7);
    };
    const easeInOutQuad = (t, b, c, d) => {
      if ((t /= d / 2) < 1) return (c / 2) * t * t + b;
      return (-c / 2) * (--t * (t - 2) - 1) + b;
    };
    return {
      visible,
      ...toRefs(state),
      isMoving,
      backToTop,
    };
  },
});
</script>



<style scoped>
.back-to-ceiling {
  position: fixed;
  display: inline-block;
  text-align: center;
  cursor: pointer;
}

.back-to-ceiling:hover {
  background: #d5dbe7;
}

.fade-enter-active,
.fade-leave-active {
  transition: opacity 0.5s;
}

.fade-enter,
.fade-leave-to {
  opacity: 0;
}

.back-to-ceiling .Icon {
  fill: #9aaabf;
  background: none;
}
</style>
